1. Field of the Invention
The present invention relates to a light scanner, a multibeam scanner, and an image forming apparatus using the same. More particularly, the present invention relates to an image forming apparatus which is suitable for use as a laser beam printer or a digital copying machine, making use of an electrophotographic process or the like, and which is used to record image information by deflecting (reflecting) light which has been emitted from a light source using a polygon mirror, serving as deflecting means, and by scanning a scan surface with the light through scanning optical means (that is, an image-forming scanning optical system). Even more particularly, the present invention relates to a multibeam scanner which achieves higher speed and higher definition by performing optical scanning operations using a plurality of light beams at the same time, and which provides a good image by reducing jitters and pitch errors.
2. Description of the Related Art
FIG. 18 is a sectional view (main scanning sectional view) of the main portion of a related multibeam scanning optical system in a main scanning direction thereof.
In FIG. 18, reference numeral 91 denotes light source means, which is, for example, a semiconductor laser array including two light-emitting points (light sources). The two light-emitting points are disposed apart from each other in the main scanning direction and a subscanning direction. Reference numeral 92 denotes a condenser lens system which includes one collimator lens and which converts two light beams that have been emitted by the light source means 91 into substantially parallel light beams or convergent light beams. Reference numeral 93 denotes a cylindrical lens which has a predetermined refractive power only in the subscanning direction. Reference numeral 94 denotes an aperture diaphragm which shapes the two light beams that have passed through the cylindrical lens 93 so that they have desired optimal shapes. Reference numeral 95 denotes deflecting means (light deflector), which is, for example, a rotating polygon mirror, and which rotates at a constant speed in the direction of arrow A by driving means 98 such as a motor. Reference numeral 96 denotes a scanning lens system (image-forming scanning optical system), serving as scanning optical means, having an fθ characteristic. The scanning lens system 96 includes two fθ lenses, a first fθ lens 96a and a second fθ lens 96b. The scanning lens system 96 has a tilt correcting function as a result of putting a location near a deflecting surface 95a of the light deflector 95 and a location near a photosensitive drum surface 97, serving as a scan surface, in a conjugate relationship within a subscanning cross-sectional plane.
In FIG. 18, the two light beams that have been emitted by the light source means 91 after being modulated in accordance with image information are converted into substantially parallel light beams or convergent light beams by the condenser lens system 92, and the converted light beams are incident upon the cylindrical lens 93. Of the light beams portions incident upon the cylindrical lens 93, those within a main scanning cross-sectional plane exit from the cylindrical lens 93 unchanged, while those within the subscanning cross-sectional plane are focused in order to form a substantially linear image (a longitudinal linear image in the main scanning direction) on the deflecting surface 95a of the light deflector 95 through the aperture diaphragm 94. Here, by the aperture diaphragm 94, the cross-sectional sizes of the light beams are limited. The two light beams that have been deflected (reflected) at the deflecting surface 95a of the light deflector 95 are focused in the form of a spot on the photosensitive drum surface 97 by the scanning lens system 96. By rotating the light deflector 95 in the direction of arrow A, the photosensitive drum surface 97 is optically scanned at a constant velocity in the direction of arrow B (main scanning direction). By this, two scanning lines are formed on the photosensitive drum surface 97, which is a recording medium, in order to record an image.
In order to record image information with high precision using this type of multibeam scanner, it is important to properly correct jitters (displacements in printing positions) and non-uniform pitches over the entire scan surface by properly focusing a plurality of light beams on the entire scan surface.
In general, when forming an image by scanning the photosensitive drum surface with light beams that have been emitted by the light source, in order to obtain a good image with high resolution, it is necessary to reduce the diameter of a light beam spot on the photosensitive drum surface and to reduce the pitch in the subscanning direction.
In order to reduce the pitch in the subscanning direction in the multibeam scanner, light source means (or a semiconductor laser array) disposed by being tilted obliquely from the main scanning direction is often used.
FIG. 19 is a sectional view (main scanning sectional view) of another related multibeam scanner of this type in a main scanning direction thereof. In FIG. 19, component parts corresponding to those shown in FIG. 18 are given the same reference numerals.
In FIG. 19, since a plurality of light-emitting points 91a and 91b of light source means 91 are disposed apart from each other by a certain distance in the main scanning direction, light beams that have exited from a condenser lens system 92 are not parallel to each other, so that they are at a certain angle from each other. Each light beam that has exited from the condenser lens system 92 is incident upon a polygon mirror 95, which is a light deflector, through a cylindrical lens 93.
At this time, the light beams cross each other at the location of an aperture diaphragm 94 disposed between the condenser lens system 92 and the polygon mirror 95, so that, by the angle of each light beam and by a distance L from a reference position of a deflecting surface 95a of the polygon mirror 95 to the aperture diaphragm 94, the interval between the light beams on the deflecting surface 95a of the polygon mirror 95 is determined (restricted). By reducing the interval between the light beams on the deflecting surface 95a of the polygon mirror 95, the light beams are properly focused on a photosensitive drum surface 97.
A multibeam scanner which satisfies such optical characteristics is disclosed in, for example, Japanese Patent Laid-Open No. 5-34613. According to this document, in the structure of the multibeam scanner, a plurality of light beams are converted into substantially parallel light beams at a condenser lens system, and the substantially parallel light beams are caused to impinge upon a polygon mirror through an aperture diaphragm. Then, by a scanning lens system, the substantially parallel light beams are led onto a scan surface. When scanning the scan surface with the plurality of light beams at the same time, the relationship among the number of light-emitting points of the light source means, the pitch in a main scanning direction, the distance from the polygon mirror to the aperture diaphragm, and the focal length of the condenser lens system is specified in order to properly focus the plurality of light beams on the scan surface.
When a multibeam scanner is used, it is necessary to properly correct jitters and pitch errors. Jitter refers to a relative displacement in positions for printing using a plurality of light beams in a main scanning direction. Pitch error refers to a deviation from a specified value (for example, 42.3 μm when printing resolution is 600 dpi) of the interval between scanning lines that are formed when a plurality of light beams are used at the same time for light scanning.
In order to reduce jitters, it is necessary to cause the light beams to reach the same location of the scanning lens system, or locations close to each other on the scanning lens system, when scanning the same location of the scan surface. This is achieved by reducing the interval between the light beams on the deflecting surface of the polygon mirror. Here, for example, a method for disposing an aperture diaphragm, serving as optimal means, near the deflecting surface of the polygon mirror is used.
However, more compact and wider-field-of-angle scanners in recent years have caused the scanning lens system and light beams deflected by the polygon mirror to be disposed near the deflecting surface of the polygon mirror, so that there is no space to dispose the aperture diaphragm. Therefore, there is a problem in that it is physically difficult to dispose the aperture diaphragm near the deflecting surface of the polygon mirror.
In general, pitch errors are corrected by causing the magnification of the scanning lens system in the subscanning direction to be constant. However, pitch errors sometimes occur by, for example, decentering of the cylindrical lens and the scanning lens system in the subscanning direction.
Here, in order to reduce how readily pitch errors are affected by decentering, it is necessary to cause the light beams to reach the same location of the scanning lens system or locations close to each other on the scanning lens system when scanning the same location of the scan surface. This can be achieved by reducing the interval between the light beams on the deflecting surface of the polygon mirror. Here, the method for disposing an aperture diaphragm, serving as optimal means, near the deflecting surface of the polygon mirror is used. (This method is the same as that used to reduce jitters.) However, the same problem as that mentioned above arises.
On the other hand, when jitters and pitch errors are reduced by disposing the aperture diaphragm near the deflecting surface of the polygon mirror, the polygon mirror becomes larger, and the scanning lens system is disposed away from the polygon mirror. Therefore, increased size of the entire scanner becomes a problem.